In some large commercial passenger aircraft, a lavatory is provided solely for crew use so they do not have to wait in line. However, this lavatory takes floor space which could be used for seats for paying passengers. Therefore, means have been sought for eliminating the crew lavatory without inconveniencing either crew or passengers.
One viable solution is for the crew to selectively lock one of the passengers' lavatory doors from the cockpit or a flight attendants' station and allow access to it by key entry only. Passengers could then use one of the other lavatories with little or no inconvenience and the crew could use the locked lavatory without waiting. However, a suitable locking system must prevent a passenger's being inadvertently locked in the lavatory without invading his privacy.
A number of automatic locking systems were considered, but none was deemed acceptable. For example, a broken light beam detector was not reliable because people might avoid breaking the beam and cabin pressurization cycles could mislocate the beam with respect to the receiver. Pressure pads in the lavatory floor are unreliable for lightweight passengers. Not all passengers lock the door when they enter a lavatory, so sensing a locked door only could possibly trap someone inside.
This invention solves the problem by an externally activated door locking system incorporating a heat and motion sensor and an electronic logic circuit controlling a secondary door lock.